1. Field of the Invention
The present invention relates to an improved control device for a charge detection circuit used as an output circuit for a CCD such as a solid-state image pick-up device.
2. Discussion of Background
There has been known a charge detection circuit called a floating diffusion amplifier as an output circuit for a CCD such as a solid-state image pick-up device.
FIG. 6 shows a conventional charge detection circuit wherein reference numeral 1 designates a CCD final gate electrode, numeral 2 designates an electric potential barrier forming gate, numeral 3 designates a diffusion region, numeral 4 designates a resetting transistor and numeral 5 designates a source follower circuit. The lower part of FIG. 6 shows the electric potential levels at respective portions. The waveform of signals which are shown in FIG. 6 at (a), (b) and (c) respectively correspond to the timing of t.sub.a, t.sub.b and t.sub.c in the timing chart shown in FIG. 7. In FIGS. 6 and 7, symbols .phi.H, .phi.R and D0 respectively represent a CCD final gate potential level, a resetting transistor gate potential level and a source follower circuit output level.
In the conventional charge detection circuit as shown in FIG. 6, an electric charge Qs transferred from the CCD is stored in the CCD final gate electrode 1 in the timing t.sub.a. In the timing t.sub.b, the electric charge Qs passes through the electric potential barrier forming gate electrode, diffuses, moves and is stored in the diffusion region 3 since the electric potential of the CCD final gate electrode 1 decreases, and the stored capacity disappeared. Then, the output D0 of the source follower circuit 5 has an output voltage of the level corresponding to the electric charge Qs. At a timing tc, the reset transistor 4 becomes in an ON state so that the electric charge Qs stored in the diffusion region 3 is discharged to an outer power source VR. Then, the preparation for storing another electric charge is completed.
In the conventional charge detection circuit described above, when, for instance, the sensitivity of the detector of a solid-state image pick-up element is increased so that an amount of electric charge becomes large, or the capacity of the diffusion region is made small in order to improve the gain of the floating diffusion amplifier, the amplitude of the output of the source follower circuit becomes large, whereby the output of the source follower circuit may reach the saturated level.